Studies on intralesional administration of lymphokine preparations to cutaneous lesions of human cancer suggest that lymphokines through their specific and/or nonspecific effects on the immune system can cause regression of human tumors. Moreover, because lymphokines are obtained from cell lines established from normal donors, and because patients in our study appeared to exhibit no ill effects from repeated lymphokine administration, it appears that lymphokines can be used "in vivo" with minimal side effects or long term difficulties. These early studies indicate that lymphokines may have a potential for treatment of cancer and a variety of other diseases. Evidence points to a central role of the immune system in host resistance to neoplasia. Besides specific antitumor immunity directed at tumor antigens, indirect effects of the immune system contribute to protection against cancer cells. We have observed the regression of tumors involving the skin of man after repeated inductions of delayed type hypersensitivity reactions to chemical haptens or microbial antigens at the tumor sites. Elements of the delayed hypersensitivity reaction that may be pertinent to to the anti-tumor effects are extravasation of plasma constituents and accumulation of a leukocytic infiltrate. In the course of the reaction the cellular composition of the leukocytic infiltrate evolves from being relatively rich in granulocytes during the early phases to a predominance of lymphocytes and macrophages during the later stages. Only a small minority of the lymphocytes of the inflammatory reaction are cells specifically sensitized to the challenge antigen, the majority being cells secondarily recruited from the blood stream to the site of the reaction presumably by the action of non-cellular mediators. Both the cellular elements and the non-cellular mediators, the lymphokines, of the infiltrate of the delayed hypersensitivity reaction may contribute to the regression of tumors at the sites of the induction of such reactions.